The present invention relates to a breather valve for a closed liquid tank and relates particularly, although not exclusively to a breather valve for milk, petrol or other liquid containing tankers.
In such tankers it is preferable to have little pressure differential between the ambient air and the air trapped within the tanker. When such a tanker is being filled the displaced air must escape or a large air pressure may build up inside. On opening the upper part of the tank e.g. to insert a "dipstick" for checking the liquid level, the rush of escaping gas from the tank, especially a petrol tank, may result in a serious fire hazard. Similarly when liquid is removed from the tank air should be permitted to enter the tank to prevent a low pressure being developed.
In the past, separate inlet and outlet breather valves have been used in an attempt to overcome such problems. These valves have usually been of the rubber mushroom or mechanical flap type. However it has been found that they can be ineffective to prevent leakage of liquid from the tank when full.